1. The Field of the Invention
The present invention relates to an active region structure for a long wavelength VCSEL incorporating antimony into its GaAsN barriers and InGaAsN quantum wells. The incorporation of Sb acts as a surfactant, thereby smoothing the growth interface. This reduces defects in the subsequent InGaAsNSb quantum well layer by eliminating seeds (steps) on which the defects may form, enhancing optical output in the 1.3 and 1.5 μm range, and improving device reliability.
2. The Relevant Technology
Vertical Cavity Surface Emitting Lasers (VCSEL) are becoming increasingly important given the proliferation of high speed data communications using fiberoptic networks. The circular profile of the output beam from VCSELs makes them ideal for coupling into fiberoptics. Further, the vertical structure of the device enables wafer scale fabrication of VCSELs, making possible two-dimensional arrays of devices for complex fiberoptic interconnection schemes as well as the incorporation of optics in integrated circuitry.
Fiberoptics for high speed data communications predominantly comprise a silica core, which has peak transmissivity and minimal dispersion at 1.55 μm and 1.3 μm respectively. Traditional GaAs VCSEL optical emission is in the 850 nm range. In order to transmit in the optimal fiberoptic wavelength range, VCSEL development has focused on decreasing the bandgap energy of GaAs in the active region of the device to impart a red shift in the optical emission. Developments toward this end include the following: the incorporation of indium in the quantum well; the addition of dilute nitrogen in the active region; improvements in the growth of multiple quantum wells through strain compensation; and the introduction of antimony in the quantum wells.
The addition of nitrogen to the InGaAs effectively decreases the bandgap energy of the quantum well by both the material composition change and the reduction in strain, extending the wavelength of the output to the 1.31 μm and potentially 1.5 um range. However the limited solubility of Nitrogen in GaAs leads to three dimensional growth and segregation into various different phases at different positions resulting in excessively broad gain spectra which is often undesirable for a VCSEL. Although lower temperatures are often used to prevent three dimensional growth, the lower temperatures result in high point defect concentrations, which decrease luminescence efficiency. Increasing the concentration of nitrogen also can cause photoluminescence to decay rapidly, which may be due to point defects. The addition of nitrogen to the barrier structure reduces the lattice constant, making it tensile relative to GaAs, and reducing the total strain of the barrier/quantum well structure. However, nitrogen may cause traps that reduce the efficiency of light emission. Further, the growth of GaAsN tends to cause three dimensional growth resulting in seeds, or steps, for segregation or dislocations in the InGaAsN quantum wells.
Current practice is to increase active region gain and thereby enhance VCSEL optical output power by growing multiple quantum wells, each stacked between barrier layers, within the active region. Strain compensation accommodates the effective stacking of barriers and quantum wells while maintaining integrity of their respective crystal structures.
FIG. 1 shows an active region structure 100 according to the related art. Conduction band 101 and valence band 102 diagrams show the bandgap features according to structure 100. Tensile GaAsN barrier layers 120, 121, and 122 provide strain compensation for compressive InGaAsN quantum wells 110, as indicated in the strain plot 140. The conduction and valence band diagrams 101 and 102 respectively represent the depth of the quantum wells 110 relative to the GaAsN barriers, according to the related art. Although the addition of nitrogen substantially shifts the wavelength of optical emissions into the near infrared, the traps that may result from nitrogen limit the performance and reliability of the device.
It has been proposed that greater and lesser concentrations of nitrogen may be accommodated in InGaAsN quantum well through the incorporation of antimony. The addition of antimony enhances the stability of the alloy which allows for more nitrogen and increases the effective critical thickness by flattening the surfaces. An additional benefit of the incorporation of Sb, due to the increase in effective critical thickness, is that it enables increased proportions of indium or reduced proportions of nitrogen, further enhancing the luminescence performance of the active region. The enhanced stability of the alloy also allows increased nitrogen concentrations, thereby enabling longer wavelengths utilizing more N, Sb, and In. Tests performed on InGaAsNSb have shown photoluminescence spectra as long as 1.6 μm exceeding that of InGaAsN at 1.3 μm.
According to current practice, the incorporation of antimony is difficult with strain compensation. Although it is currently possible to achieve alternating strains with antimony in the active region using nitrogen it can only be done with higher concentrations of a nitrogen and lower concentrations of antimony than desired.